Structure of amphotericin B aggregates as revealed by UV and CD spectroscopies

Natamycin sequesters ergosterol and interferes with substrate transport by the lysine transporter Lyp1 from yeast

Natamycin is a polyene macrolide, widely employed to treat fungal keratitis and other yeast infections as well as to protect food products against fungal molds. In contrast to other polyene macrolides, such as nystatin or amphotericin B, natamycin does not form pores in yeast membranes, and its mode of action is not well understood. Here, we have employed a variety of spectroscopic methods, computational modeling, and membrane reconstitution to study the molecular interactions of natamycin underlying its antifungal activity. We find that natamycin forms aggregates in an aqueous solution with strongly altered optical properties compared to monomeric natamycin. Interaction of natamycin with model membranes results in a concentration-dependent fluorescence increase which is more pronounced for ergosterol- compared to cholesterol-containing membranes up to 20 mol% sterol. Evidence for formation of specific ergosterol-natamycin complexes in the bilayer is provided. Using nuclear magnetic resonance (NMR) and electron spin resonance (ESR) spectroscopy, we find that natamycin sequesters sterols, thereby interfering with their well-known ability to order acyl chains in lipid bilayers. This effect is more pronounced for membranes containing the sterol of fungi, ergosterol, compared to those containing mammalian cholesterol. Natamycin interferes with ergosterol-dependent transport of lysine by the yeast transporter Lyp1, which we propose to be due to the sequestering of ergosterol, a mechanism that also affects other plasma membrane proteins. Our results provide a mechanistic explanation for the selective antifungal activity of natamycin, which can set the stage for rational design of novel polyenes in the future.

Polyene Antibiotics Physical Chemistry and Their Effect on Lipid Membranes; Impacting Biological Processes and Medical Applications

This review examined a collection of studies regarding the molecular properties of some polyene antibiotic molecules as well as their properties in solution and in particular environmental conditions. We also looked into the proposed mechanism of action of polyenes, where membrane properties play a crucial role. Given the interest in polyene antibiotics as therapeutic agents, we looked into alternative ways of reducing their collateral toxicity, including semi-synthesis of derivatives and new formulations. We follow with studies on the role of membrane structure and, finally, recent developments regarding the most important clinical applications of these compounds.

Amphotericin B assembles into seven-molecule ion channels: An NMR and molecular dynamics study

Amphotericin B, an antifungal drug with a long history of use, forms fungicidal ion-permeable channels across cell membranes. Using solid-state nuclear magnetic resonance spectroscopy and molecular dynamics simulations, we experimentally elucidated the three-dimensional structure of the molecular assemblies formed by this drug in membranes in the presence of the fungal sterol ergosterol. A stable assembly consisting of seven drug molecules was observed to form an ion conductive channel. The structure is somewhat similar to the upper half of the barrel-stave model proposed in the 1970s but substantially different in the number of molecules and in their arrangement. The present structure explains many previous findings, including structure-activity relationships of the drug, which will be useful for improving drug efficacy and reducing adverse effects.

Amphotericin B release rate is the link between drug status in the liposomal bilayer and toxicity

Amphotericin B (AmB) is an amphiphilic drug commonly formulated in liposomes and administered intravenously to treat systemic fungal infections. Recent studies on the liposomal drug product have shed light on the AmB aggregation status in the bilayer, which heat treatment (curing) modifies. Although toxicity was found related to aggregation status - loose aggregates significantly more toxic than tight aggregates - the precise mechanism linking aggregation and toxicity was not well understood. This study directly measured drug release rate from various AmB liposomal preparations made with modified curing protocols to evaluate correlations among drug aggregation state, drug release, and in vitro toxicity. UV–Vis spectroscopy of these products detected unique curing-induced changes in the UV spectral features: a ∼25 nm blue-shift of the main absorption peak (λ_max) in aqueous buffer and a decrease in the OD₃₄₆/OD₃₂₂ ratio upon thermal curing, reflecting tighter aggregation. In vitro release testing (IVRT) data showed, by applying and fitting first-order release kinetic models for one or two pools, that curing impacts two significant changes: a 3–5-fold drop in the overall drug release rate and a ten-fold decrease in the ratio between the loosely aggregated and the tightly aggregated, more thermodynamically stable drug pool. The kinetic data thus corroborated the trend independently deduced from the UV–Vis spectral data. The in vitro toxicity assay indicated a decreased toxicity with curing, as shown by the significantly increased concentration, causing half-maximal potassium release (TC₅₀). The data suggest that the release of AmB requires dissociation of the tight complexes within the bilayer and that the reduced toxicity relates to this slower rate of dissociation. This study demonstrates the relationship between AmB aggregation status within the lipid bilayer and drug release (directly measured rate constants), providing a mechanistic link between aggregation status and in vitro toxicity in the liposomal formulations.

Influence of gamma radiation on Amphotericin B incorporated in PVP hydrogel as an alternative treatment for cutaneous leishmaniosis

Amphotericin B (Amph-B) is an antifungal drug used intravenously for the treatment of leishmaniasis. Side-effects from Amph-B treatment can arise such as cardiac arrhythmia and renal dysfunctions, which will lead to discontinuation of treatment. Unfortunately, patients in endemic countries do not have access to alternative therapies. The objective of this study was to analyze the effects of Cobalt-60 gamma irradiation on crosslinking polymeric hydrogels (Hydg) and the incorporation of Amph-B into the gel as a controlled-release drug delivery alternative. Polyvinylpyrrolidone (PVP)/Amph-B solutions were irradiated with 15 kGy at 0 °C and 25 °C. The drug's stability was ascertained by UV-visible spectrometry, liquid chromatography/mass spectrometry and proton nuclear magnetic resonance. Irradiated Hydg/Amph-B achieved similar stability to the standard Amph-B solution and was enough to promote hydrogel crosslinking. In vitro trials were carried out to ensure Amph-B was still biologically active after irradiation. The results from flow cytometry and MTT assay show that Amph-B had an IC₅₀ = 16.7 nM. A combination of Hydg at 1.324 gmL^-1 and Amph-B at 25.1 nM for 24 h lead to the greatest inhibition of L. amazonensis promastigotes, and could be used as an alternative treatment method for cutaneous leishmaniosis.

Thermodynamics and kinetics of amphotericin B self-association in aqueous solution characterized in molecular detail

Amphotericin B (AmB) is a potent but toxic drug commonly used to treat systemic mycoses. Its efficiency as a therapeutic agent depends on its ability to discriminate between mammalian and fungal cell membranes. The association of AmB monomers in an aqueous environment plays an important role in drug selectivity, as oligomers formed prior to membrane insertion - presumably dimers - are believed to act differently on fungal (ergosterol-rich) and mammalian (cholesterol-rich) membranes. In this work, we investigate the initial steps of AmB self-association by studying the structural, thermodynamic and spectral properties of AmB dimers in aqueous medium using molecular dynamics simulations. Our results show that in water, the hydrophobic aggregation of AmB monomers yields almost equiprobable populations of parallel and antiparallel dimers that rapidly interconvert into each other, and the dipole-dipole interaction between zwitterionic head groups plays a minor role in determining the drug's tendency for self-aggregation. A simulation of circular dichroism (CD) spectra indicates that in experimental measurements, the signature CD spectrum of AmB aggregates should be attributed to higher-order oligomers rather than dimers. Finally, we suggest that oligomerization can impair the selectivity of AmB molecules for fungal membranes by increasing their hydrophobic drive for non-specific membrane insertion.

Effect of imidazolium room-temperature ionic liquids on aggregation of amphotericin B: a circular dichroism study

The aggregate–monomer equilibrium of amphotericin B could be controlled by the structure of the room-temperature ionic liquid.

Circular dichroism studies on intermolecular interactions of amphotericin B in ionic liquid-rich environments

Aggregation of amphotericin B (AmB) in an ionic liquid-rich environment was investigated using circular dichroism (CD) spectroscopy. It was found that nature of the ionic liquids' anion had a strong impact not only on the aggregation of AmB, but more importantly on the nature of AmB aggregates, as observed in the asymmetry of the exciton couplet of the aggregate in CD spectra. Unique CD signals for AmB aggregates were observed in three different 1-butyl-3-methylimidazolium ionic liquid solutions: [C4 -mim]Br favored the formation of AmB aggregates that were similar to those found in water, whereas [C4 -mim]BF4 and [C4 -mim]NO3 produced AmB aggregates that were different from each other and those found in water. The obtained results suggest that the designer solvent ability of ionic liquids could be expanded to address numerous intermolecular processes.

Spectroscopic studies of amphotericin B-Cu²+ complexes

The aim of this research is to investigate amphotericin B (AmB)-Cu(2+) complexes in aqueous solution at different pH values. Electronic absorption, circular dichroism (CD), Raman and FTIR spectroscopies were used in this study. We found that different concentrations of AmB and Cu(2+) ions in solution leads to formation of complexes with stoichiometry of 2:1 and 1:1. The formation of AmB-Cu(2+) complexes at physiological pH values is accompanied by changes of the molecular organization of AmB especially disaggregation. These observed effects might be significant from a medical point of view.

Synthesis-enabled functional group deletions reveal key underpinnings of amphotericin B ion channel and antifungal activities

Amphotericin B is the archetype for small molecules that form transmembrane ion channels. However, despite extensive study for more than five decades, even the most basic features of this channel structure and its contributions to the antifungal activities of this natural product have remained unclear. We herein report that a powerful series of functional group-deficient probes have revealed many key underpinnings of the ion channel and antifungal activities of amphotericin B. Specifically, in stark contrast to two leading models, polar interactions between mycosamine and carboxylic acid appendages on neighboring amphotericin B molecules are not required for ion channel formation, nor are these functional groups required for binding to phospholipid bilayers. Alternatively, consistent with a previously unconfirmed third hypothesis, the mycosamine sugar is strictly required for promoting a direct binding interaction between amphotericin B and ergosterol. The same is true for cholesterol. Synthetically deleting this appendage also completely abolishes ion channel and antifungal activities. All of these results are consistent with the conclusion that a mycosamine-mediated direct binding interaction between amphotericin B and ergosterol is required for both forming ion channels and killing yeast cells. The enhanced understanding of amphotericin B function derived from these synthesis-enabled studies has helped set the stage for the more effective harnessing of the remarkable ion channel-forming capacity of this prototypical small molecule natural product.

Nanodisks derived from amphotericin B lipid complex

The goal of this study was to determine the effect of apolipoproteins on Amphotericin B lipid complex (ABLC). We report that incubation of ABLC with recombinant human apolipoprotein A-I (apoA-I) induces solubilization of ABLC by transforming the micron sized phospholipid/AMB assemblies into discrete nanoscale disk-shaped complexes termed nanodisks (ND). ApoA-I induced changes in ABLC solubility and morphology were monitored by spectroscopy and electron microscopy. AMB efficacy was evaluated in yeast and pathogenic fungi growth inhibition assays and the effect of AMB formulation on cell toxicity was assessed in cultured Hep3B cells. AMB associated with ND were more efficiently nebulized than AMB associated with ABLC. Thus, transformation of ABLC into ND preserves the potent biological activity of AMB as well as the reduced toxicity of the ABLC formulation. ABLC derived AMB-ND offer advantages over conventional ABLC in terms of stability, storage, nebulization efficiency and provides an intrinsic "handle" for tissue specific targeting via genetic engineering of its protein component.

Influence of the vehicle on the properties and efficacy of microparticles containing amphotericin B

New microparticles containing amphotericin B (AMB) have been developed and manufactured by spray drying. To this end albumin, polylactic-co-glycolic acids (PLGA) and poly(sebacic anhydride) have been employed as drug carriers. The selection of the solvent used to disperse the drug and the vehicle before spray drying was critical on production yields and physical properties of the microparticles. Once particle size, morphology and dispersability in some aqueous media were shown to be acceptable for an intravenous administration, in vivo efficacy was evaluated and compared with the reference medicine Fungizone. Microparticles prepared with albumin, albumin heated at a high temperature, some kinds of PLGA or polyanhydride, as well as Fungizone, were tested in an experimental hamster model of infection with Leishmania infantum, by evaluating the evolution of parasitic burdens in spleen, liver and antibody responses. After the injection of three doses corresponding to 2 mg of AMB per kilogram each, diverse reactions were reported depending on the vehicle. The best dispersability, reduction of parasites and antibody response were achieved when the treatment was performed with AMB in albumin microspheres.

Amphotericin B Formulations and Drug Targeting

Amphotericin B is a low-soluble polyene antibiotic which is able to self-aggregate. The aggregation state can modify its activity and pharmacokinetical characteristics. In spite of its high toxicity it is still widely employed for the treatment of systemic fungal infections and parasitic disease and different formulations are marketed. Some of these formulations, such as liposomal formulations, can be considered as classical examples of drug targeting. The pharmacokinetics, toxicity and activity are clearly dependent on the type of amphotericin B formulation. New drug delivery systems such as liposomes, nanospheres and microspheres can result in higher concentrations of AMB in the liver and spleen, but lower concentrations in kidney and lungs, so decreasing its toxicity. Moreover, the administration of these drug delivery systems can enhance the drug accessibility to organs and tissues (e.g., bone marrow) otherwise inaccessible to the free drug. During the last few years, new AMB formulations (AmBisome, Abelcet, and Amphotec) with an improved efficacy/toxicity ratio have been marketed. This review compares the different formulations of amphotericin B in terms of pharmacokinetics, toxicity and activity and discusses the possible drug targeting effect of some of these new formulations.

Mixed micellar nanoparticle of amphotericin B and poly styrene-block-poly ethylene oxide reduces nephrotoxicity but retains antifungal activity

Mixed micellar nanoparticle consisting of amphotericin B (AmB) and poly styrene-block-poly ethylene oxide (PS-block-PEO) was prepared by high pressure homogenizer. Nephrotoxicity of the nanoparticle was investigated along with antifungal activity and self-aggregation status of the drug in the nanoparticle. Nephrotoxicity was markedly reduced when AmB was intravenously administered to rats as mixed micellar nanoparticle with PS-block-PEO in terms of transmission electron microscopy of tubular cells and creatinine clearance. Antifungal activity of AmB was not altered when the drug was in the form of mixed micellar nanoparticle compared to both conventional formulation and AmB micelle treated by same procedure without PS-block-PEO. Self-aggregation status of AmB molecules revealed monomeric in the mixed micellar nanoparticle with PS-block-PEO up to the therapeutic level of the drug (1-3 mM). The reduced nephrotoxicity of AmB in mixed micellar nanoparticle may be associated with the existence of the drug as monomeric form in the nanoparticle. Based on our result, formulation of AmB as mixed micellar nanoparticle with PS-block-PEO may be a promising alternative for the treatment of fungal diseases in patients who are at risk of renal dysfunction.

Fluorescence of amphotericin B-deoxycholate (fungizone) monomers and aggregates and the effect of heat-treatment

Fluorescence excitation and emission spectra are reported for the polyene macrolide antifungal agent Amphotericin B formulated as micellar dispersion Fungizone (FZ) and its modified counterpart heat-treated Fungizone. The addition of sodium dodecyl sulfate or sodium deoxycholate surfactant to modulate the aggregation state of Amphotericin B confirms that the monomer and dimer states have different fluorescence spectra. Energy transfer from excited dimer to monomer is observed. Both FZ and heat-treated FZ (HTFZ) show expected S1 --> S0 fluorescence emission as well as anti-Kasha fluorescence emission from the S2 state. The excitation and S1 --> S0 emission spectra of HTFZ are similar to those of FZ, while the S2 --> S0 fluorescence differs in intensity between them. The variation in the rate constant for internal conversion from S2 to S1 as the surfactant concentration is increased differs for FZ and HTFZ; we propose that this may form a new basis for examining the super-aggregated character of AmB preparations. FZ and HTFZ have a similar stability to disaggregation by added sodium dodecyl sulfate surfactant. These findings provide the groundwork for future fluorescence characterization of FZ or HTFZ interactions with cell membranes.

Arabinogalactan protein from Arachis hypogaea: Role as carrier in drug-formulations

Arabinogalactan protein (AGP) a highly water-soluble glyco-conjugate from groundnut (Arachis hypogaea L.) seedling was isolated and purified by precipitation with beta-glucosyl Yariv reagent. Quantification of AGP was done by gel diffusion assay. Purified AGP was conjugated to amphotericin-B (AmB) by Schiff base reaction at pH 11.0, with aim to prepare a water-injectable lesser toxic AGP-AmB conjugate without affecting AmB antifungal potential. The AGP-AmB conjugate antifungal activity was assayed by serial broth dilution and disc method against several Candida albicans clinical isolates. Both AmB and AGP-AmB showed similar MICs and MFCs activities, indicating that AGP do not reduced the antifungal activity of AmB. However, the in vitro and in vivo toxicity assays revealed that AGP-AmB conjugate was lesser toxic than AmB, as high MTD (45 mg/kg body weight) was observed. It is suggested that AGP could be a potent carrier in AmB formulation, which may result in effective treatment of fungal infections.

Encapsulation of amphotericin B in poly(ethylene glycol)-block-poly(epsilon-caprolactone-co-trimethylenecarbonate) polymeric micelles

The aim of this work was to evaluate the potential of self-assembling poly(ethyleneglycol)(750)-block-poly(epsilon-caprolactone-co-trimethylenecarbonate)(4500) 50/50 copolymers (PEG-p(CL-co-TMC)) to solubilize amphotericin B in polymeric micelles and to disaggregate the drug to the less toxic monomeric form. Amphotericin B was encapsulated in the micelles upon dilution of a mixture of the liquid polymer and the drug in water. Its solubility was increased by two orders of magnitude depending on polymer concentration. The aggregation state of amphotericin B was decreased by PEG-p(CL-co-TMC). The preparation method and the loading of the polymeric micelles influenced it. The antifungal activity of the drug was reduced by encapsulation in the polymeric micelles whereas the onset of amphotericin B-induced hemolysis was delayed. PEG-p(CL-co-TMC) micelles could be an easy method for amphotericin B encapsulation.

Spectroscopic studies of amphotericin B solubilized in nanoscale bilayer membranes

Nanodisks (ND) are discrete nanometer scale phospholipid bilayers whose perimeter is circumscribed by amphipathic apolipoproteins. The membranous environment of ND serves as a matrix for solubilizing the polyene antibiotic amphotericin B (AMB). The spectral properties of AMB in ND are dependent upon AMB concentration. Whereas AMB-ND prepared at a concentration of 2.5 mg AMB per 10 mg phospholipid are consistent with AMB self association in the ND membrane environment, AMB-ND prepared at 0.25 or 0.025 mg AMB per 10 mg phospholipid give rise to spectra reminiscent of AMB in organic solvent. Incubation of ND prepared at a phospholipid/AMB ratio of 400:1 (w/w) at 37 degrees C for 1 h induced a shift in absorbance and near UV circular dichroism spectra consistent with antibiotic self-association. The kinetics of this spectral transition were investigated as a function of incubation temperature. While no change in A388 nm occurred in incubations at 20 degrees C, a time-dependent decrease in A388 nm was observed at 25, 30 and 37 degrees C. Inclusion of ergosterol in the ND membrane attenuated temperature-induced AMB spectral changes. In Saccharomyces cerevisiae growth inhibition assays, ND containing self associated AMB were somewhat less effective than ND possessing a greater proportion of monomeric AMB. On the other hand, inclusion of ergosterol or cholesterol in the ND particle did not alter the growth inhibition properties of AMB-ND. The miniature membrane environment of ND provides a novel milieu for solubilization and characterization of lipophilic biomolecules.

Amphotericin B molecular organization as an essential factor to improve activity/toxicity ratio in the treatment of visceral leishmaniasis

An in vivo study has been performed in order to determine the influence of amphotericin B (AMB) molecular organization on the toxicity and activity of this drug in the treatment of experimental visceral leishmaniasis. Three formulations with similar composition but different drug molecular self-association in aqueous media were prepared. Acute toxicity was evaluated by injecting them in healthy hamsters. Sub-acute toxicity and efficacy were studied administering them to animals previously infected with Leishmania infantum. The preparation with drug molecules completely dissolved into monomers (formulation "C") and produced the highest acute toxicity. The formulation whose AMB molecules were disposed as non-water-soluble multi-aggregates (formulation "B") proved to provide the lowest acute toxicity. This formula also showed an improved activity, mainly in the liver, if compared with the third tested formulation containing AMB molecules disposed as smaller dimerical "water-soluble" aggregates (formulation "A"). As a conclusion, molecular aggregation in biological media should be an important factor to consider when researching or optimizing medicines containing AMB. The liberation of molecules as large dispersed non-water-soluble multi-aggregates seems to improve the narrow therapeutic margin attached to the use of this drug.

Membrane-permeabilizing activities of amphidinol 3, polyene-polyhydroxy antifungal from a marine dinoflagellate

Amphidinols, which are polyene-polyhydroxy metabolites produced by the marine dinoflagellate Amphidinium klebsii, possess potent antifungal and hemolytic activities. The membrane permeabilizing actions of amphidinol 3, the most potent homologue, were compared with those of polyene antibiotics, amphotericin B (AmB) and filipin, in hemolytic tests, 23Na nuclear magnetic resonance (NMR)-based membrane permeabilizing assays, and UV spectroscopy for liposome-bound forms. In Na+ flux experiments using large unilamellar vesicles (LUVs), ion efflux by amphidinol 3 was inhibited by cholesterol or ergosterol, which was opposed to previous results [J. Mar. Biotechnol., 5 (1997) 124]. When the effect of the agents on the size of vesicles was examined by light scattering experiments, amphidinol 3 did not significantly alter their size while filipin and synthetic detergent Triton X-100 did. The observations implied that the activity of amphidinol 3 was mainly due to formation of large pores/lesions in liposomes rather than detergent-like disruption of membrane. The pore/lesion size was estimated to be 2.0-2.9 nm in diameter on the basis of osmotic protection experiments using blood cells. The UV spectra in liposomes, which revealed the close interaction of polyene moieties in a lipid bilayer, further implied that the membrane activity of amphidinol 3 is caused by the molecular assemblage formed in biomembrane. These results disclose that amphidinol 3 is one of few non-ionic compounds that possess potent membrane permeabilizing activity with non-detergent mechanism.

Formation of natamycin:cyclodextrin inclusion complexes and their characterization

Natamycin is a broad spectrum antimycotic with very low water solubility, which is used to extend the shelf life of shredded cheese products. beta-Cyclodextrin (beta-CD), hydroxypropyl beta-cyclodextrin (HP beta-CD), and gamma-cyclodextrin (gamma-CD) were found to form inclusion complexes with natamycin in aqueous solution. The increase in solubility of natamycin with added beta-CD was observed to be linear (type A(L) phase solubility diagram). The 1:1 stability constant of natamycin:beta-CD complex was estimated from its phase solubility diagram to be 1010 M(-1). The phase solubility diagrams of both gamma-CD and HP beta-CD exhibited negative deviation from linearity (type A(N) diagram) and, therefore, did not allow the estimation of binding constants. The water solubility of natamycin was increased 16-fold, 73-fold, and 152-fold with beta-CD, gamma-CD, and HP beta-CD, respectively. The natamycin:CD inclusion complexes resulted in in vitro antifungal activity nearly equivalent to that of natamycin in its free state.

Membrane permeabilizing activity of amphotericin B is affected by chain length of phosphatidylcholine added as minor constituent

The effect of acyl-chain length of phospholipid on the membrane permeabilizing activity of amphotericin B (AmB) was examined using egg phosphatidylcholine (eggPC) liposomes containing 5% or 20% phosphatidylcholine with various lengths of fatty acyl chains from C(10) to C(18); 1,2-dicapryloyl-sn-glycero-3-phosphocholine (DCPC), 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC). The membrane activity of AmB was evaluated by two methods; the drug was added to a liposome suspension (added-via-aqua), or mixed with lipids prior to liposome preparation (mixed-with-lipid). In both cases, K(+) influx by AmB was measured as pH change inside liposomes by 31P-NMR. The C(10) and C(12) acyl phospholipids markedly enhanced the activity of AmB, the C(14) and C(16) lipids virtually showed no effect, and the C(18) lipid was inhibitory to the AmB's action. Clear distinction between the C(12) and C(14) lipids, which differ only in acyl chains by two carbons, implies that molecular interaction between phospholipid and AmB is partly due to the matching of their hydrophobic length.

Amphotericin B/emulsion admixture interactions: an approach concerning the reduction of amphotericin B toxicity

Mixing Fungizone with a fat emulsion used for nutritional purpose (Intralipid or Lipofundin ) was reported to decrease Amphotericin B (AmB) toxicity in clinical use. In an effort to understand the reason for this phenomenon, spectral and morphological analyses were done for the Fungizone and Fungizone /Lipofundin admixture (FLmix). The absorption spectra analyses showed that not only Fungizone but also FLmix presented spectra that were concentration dependent. Moreover, the spectra of FLmix remained stable until the concentration of 5 x 10(-7) M, and only at 5 x 10(-8) M did they become similar in shape to the Fungizone spectra. Morphological studies revealed that even though emulsion droplets with or without Fungizone presented the same particle size, the former was less electron dense compared with Lipofundin alone. These results suggest a kind of association between Fungizoneand Lipofundin that remains over the whole range of concentrations. This hypothesis was confirmed by in vitro studies in which FLmix presented an important selectivity against human and fungal cells compared with Fungizone. These findings suggest that parenteral emulsions should be able to reduce the AmB toxicity probably by changing the AmB self-association state by binding it with emulsion droplets.

Synthesis and characterization of novel water soluble amphotericin B-arabinogalactan conjugates

The coupling of amphotericin B (AmB), a water-insoluble antifungal agent, to arabinogalactan (AG) via an imine or amine bond was systematically investigated. AG was oxidized using potassium periodate, purified from the oxidizing agent using ion-exchange chromatography, and reacted with AmB to form the Schiff base. The Schiff base was reduced to the amine using borohydride. All reactions took place in aqueous media. The purification of the oxidized AG from the oxidizing agent was essential to prevent oxidative degradation of AmB at the coupling step. We investigated the effects of AmB to AG ratio, buffer type, and reaction pH on the reaction yield, molecular weight, conjugate activity against pathogenic yeast and hemolytic activity. The optimum coupling conditions were buffer borate 0.1 M, pH 11 at room temperature for 48 h. Lower toxicity in vivo was achieved by using low-pressure gel permeation chromatography and applying the solution of AmB-AG conjugate through a Sephadex column. Both amine and imine AmB-AG conjugates were soluble in water and exhibited improved stability in aqueous solutions as compared to the unbound drug. The conjugates showed comparable minimum inhibitory concentration (MIC) values against Candida albicans. The conjugates were about 60 times less hemolytic against sheep erythrocytes than the free drug, and about 40 times less toxic in BALB/c mice.

Thermodynamic and spectroscopic features of the behavior of amphotericin B in aqueous medium

The interaction between amphotericin B molecules in aqueous medium solution was studied using absorption and circular dichroism approaches. The results showed that at concentrations below 1 microM of amphotericin B, an equilibrium between the monomer and aggregate occurred with a constant of approximately 0.6x10(6) M(-1). The aggregate formation constant was dependent on the experimental conditions of the medium: its value increased at acidic pH values, while alkaline medium induced the equilibrium displacement to the monomer formation. Either neutral salts or chaotropic agents such as urea prevented the formation of the aggregate. The presence of net electrical charge on the amine and carboxyl groups plays a role in the thermodynamic stability of the aggregate. A hydrophobic effect was also found between the monomer form and the water molecules of neighbours. In the aggregate formation water molecules were released contributing to an increase in the entropic change.

In vitro dissociation of antifungal efficacy and toxicity for amphotericin B-loaded poly(ethylene oxide)-block-poly(beta benzyl L aspartate) micelles

Amphotericin B (AmB) is a membrane-active drug used frequently for the treatment of systemic fungal diseases. Limitations for the use of AmB include poor water solubility and potential for serious systemic toxicities. Recently, it has been demonstrated that the aggregation state of AmB is a determinant factor for toxicity. To increase its therapeutic index, AmB has been solubilized in micelles based on poly(ethylene oxide)-block-poly(beta-benzyl-l-aspartate) (PEO-block-PBLA), using a dialysis method of drug loading. The aggregation state of AmB has been investigated by electronic absorption spectroscopy. AmB loaded in PEO-block-PBLA micelles is non-hemolytic for concentrations up to 15 microgram/ml. AmB as Fungizone(R) initiates hemolysis at 1.0 microgram/ml. The onset of hemolysis correlates with the respective critical aggregation concentrations (CACs) of AmB. The antifungal activity of the AmB-loaded PEO-block-PBLA micelles is four to eight times higher than Fungizone(R) in terms of minimal inhibitory concentrations (MICs). PEO-block-PBLA has no antifungal activity for concentrations up to 200 microgram/ml. The basis for the increase in antifungal activity of AmB-loaded PEO-block-PBLA micelles is unclear, but may be related to a stabilizing effect of the polymeric micelles against auto-oxidation of the AmB heptaene moiety or alternatively, an enhancement in membrane perturbation of fungal cells.

Heat-induced superaggregation of amphotericin B reduces its in vitro toxicity: a new way to improve its therapeutic index

Superaggregation of amphotericin B (AmB) was previously shown to occur upon heating of solutions at 70 degrees C. In the present study, we demonstrate that heat pretreatment of Fungizone (deoxycholate salt of AmB [AmB-DOC]) solutions induces a drastic decrease in the in vitro toxicity of this antibiotic. Heated AmB-DOC colloidal solutions, which mainly contained superaggregated and monomeric forms of the antibiotic, were strongly less hemolytic than unheated solutions (aggregates and monomers). Thermal pretreatment of AmB-DOC solutions also reduced the toxicity to the cell line HT29, as deduced from two simultaneous cell viability assays (3-4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and lactate dehydrogenase release). These heated colloidal solutions were only slightly less efficient than the unheated ones at inhibiting the growth of Candida albicans cells in vitro. Such results suggest that mild heat treatment of AmB-DOC solutions could provide a new and simple solution for improving the therapeutic index of this antifungal agent by reducing its toxicity to mammalian cells.

Physico-chemical properties of the heat-induced ‘superaggregates’ of amphotericin B

The aggregation state of amphotericin B (AmB) was previously reported to modulate its therapeutic efficiency. As a preliminary study to test the biological effects of 'superaggregates' generated by heat treatment, we present spectroscopic data related to their formation in aqueous solutions. Drastic changes in the AmB aggregation state in water were shown to occur on heating at 50-60 degrees C. The concentration of the aggregates formed at high (A(t)) or room (A) temperature, and the concentration of the monomeric form (M) of AmB were calculated by processing absorption data. The thermally induced conversion from A to A(t) depends on the AmB concentration. Rayleigh scattering measurements suggest that the A(t) aggregates are larger than the A aggregates. At room temperature, the condensation rate of A with M-leading to the 'superaggregated' form A(t)-was slower and depended on the concentration of M. The superaggregated species A(t) was shown to be the most chemically stable species. Physico-chemical properties of these superaggregates are discussed as a potential new solution to improve the therapeutic efficacy of AmB.

Molecular dynamics of amphotericin B. II. Dimer in water

Molecular dynamics simulations were performed for a dimer of the antifungal antibiotic, amphotericin B, in water. In the first step of the work three appropriately selected versions of the dimer structure were taken into consideration. In each version antibiotic molecules were placed antiparallel with polar and ionizable groups outside the hydrophobic core formed by polyene chromophores. During short dynamic simulations versions of the dimer structure were compared in respect of the energy of dimerization. The highest energy was observed for the structure in which polyene chromophores superimposed each other as much as possible and this version was subjected to the main simulation. The analysis of 66 snapshot geometries stored during 33 ps dynamic trajectory allowed us to draw three main conclusions: (i) the relative orientation of the amino-sugar moiety and chromophore as well as conformation of the antibiotic macrolide ring were different in both molecules and could exhibit dynamic changes, (ii) the dimer structure exhibited intrinsic asymmetry which could be responsible for characteristic circular dichroism spectra of the aggregated form of the antibiotic, (iii) relatively high stability of the dimer structure resulted not only from hydrophobic interactions between chromophores but also from hydrogen bonds networks that were observed around polar terminals of antibiotic molecules. Implications of these features of the dimer structure for its susceptibility on the ionic state of carboxyl and/or amino groups are also discussed.

Theoretical study of the self-association of amphotericin B

The aim of this present work is the study of self-association of amphotericin B (AmB) at a molecular levels, because of its importance in the toxicity of this antibiotic. Molecular mechanics calculations have been performed considering different conformations of the polar head of AmB, the two most stable ones we have determined (B and C) and the one issued from the X-ray data. Our calculations have shown that both head-to-head and head-to-tail stable dimers were found within an energy range between -30 and -40 kcal/mol, the very stable head-to-head dimer with the polar head within C conformation having an energy of -46.8 kcal/mol. We have shown that both electrostatic and Van der Waals terms contribute to the total interaction energy but their relative weight depends on the conformation of the polar head and on the head-to-head and head-to-tail structures involved in the dimer. Thus the electrostatic contribution does no particularly stabilize the head-to-tail dimer. Furthermore an explicit calculation of the dipole moment in the ground state of AmB has disproved the current assertion upon the greatest stabilization of head-to-tail dimers by electrostatic dipole-dipole interaction. Among all the dimers we have calculated, we have found a group denoted G1 with a geometrical structure consistent with absorption data, namely a blue-shift of the dimer main absorption band with regard to the monomer one. In this group G1 we have found two isoenergetic (-38.8 kcal/mol) very stable head-to-head and head-to-tail dimers. We have found that, as a rule, the self-association of AmB in dimers is more favourable than the complexation with the cholesterol and, in a less extent, with the ergosterol. It seems that these features could be also observed for some trimers, that we have roughly calculated.

Filipin and its interaction with cholesterol in aqueous media studied using static and dynamic light scattering

Aggregation of filipin in aqueous medium and filipin-induced changes in cholesterol micelles have been studied using intensity and dynamic light scattering. The dependencies of filipin aggregate dimensions on concentration, solvent, and temperature were studied, and revealed that the aggregates do not have a well-defined geometry, i.e., a critical micelle concentration cannot be detected and stable structures are not formed. The aggregates are of size Rg approximately 110 nm and Rh approximately 63 nm, referring to the radius of gyration and hydrodynamic radius, respectively. In the concentration range studied (1 microM < C < 30 microM), a low molecular weight species (monomer/dimer) is always present together with the aggregates. In ethanol/water mixtures, large (Rg approximately 500 nm), narrow distribution aggregates are formed in the water volume fraction range 0.45 < phi H2O < 0.65. Aggregation also occurs on changing the temperature; In the range 7-37 degrees C, smaller aggregates (10-30 nm form and the process is only partially reversible. No pronounced effect of filipin on the structure of the cholesterol micelles was observed (a small increase in Rg and Rh is noted). These results rule out any "specificity" for the filipin interactions with cholesterol, which has been considered a key event in the filipin biochemical mode of action. A reevaluation of this question is suggested and some alternatives are advanced.

Effect of aggregation on the kinetics of autoxidation of the polyene antibiotic amphotericin B

We have previously studied the autoxidation of the polyene antibiotic amphotericin B (AB). In this paper we describe the dependence of the kinetics of autoxidation on the aggregation state of the antibiotic. Autoxidation, which is involved in drug inactivation and has been suggested to play a role in the mechanism of drug action, was assessed through the reaction of formed radicals with the spin label Tempol (2,2,6,6-tetramethyl-4-hydroxy-N-oxylpiperidine) by following the loss of the electron spin resonance signal, as previously described, and by oxygen consumption. Two types of AB (I and II) were used, the former being obtained by further purification of the latter. The kinetics of autoxidation were compared for aggregates formed by the antibiotic. Differences in aggregation state for both type I and type II AB were observed between monomeric, borax-complexed, and preparations in water containing variable proportions of dimethyl sulfoxide (DMSO) by optical absorption and circular dichroism (CD) spectra. On the other hand, although the suspensions of type I and type II AB in water-10% DMSO did not differ in their optical properties, they could be distinguished by quasielastic light scattering experiments, type II yielding smaller aggregates. It is proposed that the lack of difference in optical and CD spectra are due to the similarity of the microenvironments in both aggregates. In contrast, the borax complex of both type I and type II AB yielded similar optical and CD spectra and quasielastic light scattering behavior, indicating that complexation led to similar aggregates.(ABSTRACT TRUNCATED AT 250 WORDS)

A kinetic method for measuring functional delivery of amphotericin B by drug delivery systems

The human toxicity of amphotericin B can be considerably reduced by associating the drug with liposomes of varying lipid compositions. Some lipid compositions are much more effective than others. We show that a simple kinetic fluorescence assay using pyranine as an indirect probe of amphotericin-induced K+ currents may be used to study different liposomal drug delivery systems in vitro. We find that lipid mixtures composed of DMPC/DMPG/amphotericin at a 7:3:1 mole ratio show very slow functional delivery with a preference for ergosterol over cholesterol-containing membrane vesicles. On the other hand, amphotericin delivered from egg phosphatidylcholine liposomes lead to 100-fold increases in K+ leakage at one-fifth the amphotericin concentration of the 7:3:1 system. The egg phosphatidylcholine system as well as micellar amphotericin also show a slight selectivity towards cholesterol-containing vesicles over ergosterol. These results are consistent with previous clinical and in vitro cellular studies and this technique may prove valuable in screening of other delivery systems.

Characterization and time dependence of amphotericin B: deoxycholate aggregation by quasielastic light scattering

Quasielastic light scattering measurements of amphotericin B (AB):deoxycholate (DOC) preparations provided information about particle size and aggregation as a function of concentration. The data allowed the time dependence of the aggregation to be followed and indicated that the initial rates of the change in average equivalent hydrodynamic diameter increased with decreasing concentration. The results extend the model proposed by Lamy-Freund and co-workers, which describes AB:DOC systems as consisting of AB:DOC mixed aggregates co-existing with pure DOC micelles. Although the AB:DOC aggregates are unstable at all concentrations studied, the rate of aggregation increases by three orders of magnitude as the concentration is reduced from 20 mM (DOC concentration) to the concentration region of DOC micellization. These results are in agreement with the different distribution of AB and DOC in the body of experimental animals, and may be of relevance for the understanding of the serious toxic effects of AB.